Deposition of buffer layers on textured metal surfaces

ABSTRACT

A method of making a multilayer article includes depositing a first material on the surface of a metal substrate to form a seed layer of the first material, the first material being deposited under reducing conditions relative to the metal substrate, and then epitaxially depositing a second material on a surface of the seed layer, wherein the second material is deposited from a solution-based precursor under second conditions that are more oxidizing than the reducing conditions used in the deposition of the first material.

FIELD OF THE INVENTION

The invention generally relates to multilayer articles and methods ofmaking such articles, and in particular, relates to multilayer articlescontaining epitaxially grown layers. The invention more particularlyrelates to the growth of an epitaxial metal oxide buffer layer(s) onmetal substrates.

BACKGROUND OF THE INVENTION

Multi-layer articles can be used in a variety of applications. Biaxiallytextured metal oxide buffer layers on metal substrates are potentiallyuseful in electrical and/or electronic devices where an electricallyactive layer is deposited on the buffer layer. The electrically activelayer may be a superconductor, a semiconductor, or a ferroelectricmaterial. Typically, such articles 10 include a metal substrate 11,buffer layer(s) 12, and an active layer 13, e.g., a superconductor, asillustrated in FIG. 1. The metal substrate, such as Ni, Ag, or Nialloys, provides flexibility for the article and can be fabricated overlong lengths and large areas. Metal oxide layers, such as LaAlO₃, CeO₂,or yttria-stabilized zirconia (YSZ), make up the next layer and serve aschemical barriers between the metal substrate and the active layer. Thebuffer layer(s) can be more resistant to oxidation than the substrateand can reduce the diffusion of chemical species between the substrateand the superconductor layer. Moreover, the buffer layer(s) can have acoefficient of thermal expansion that is well matched with thesuperconductor material.

To achieve high critical current densities in the article, thesuperconducting material is biaxially textured and strongly linked. Asused herein, “biaxially textured” refers to a surface for which thecrystal grains are in close alignment with a direction in the plane ofthe surface and a direction perpendicular to the surface. One type ofbiaxially textured surface is a cube textured surface, in which thecrystal grains are also in close alignment with a directionperpendicular to the surface. Examples of cube textured surfaces includethe (100)[001] and (100)[011] surfaces, and an example of a biaxiallytextured surface is the (113)[211] surface.

Typically, the buffer layer is an epitaxial layer, that is, itscrystallographic orientation is directly related to the crystallographicorientation of the surface onto which the buffer layer is deposited. Forexample, in a multi-layer superconductor having a substrate, anepitaxial buffer layer and an epitaxial layer of superconductormaterial, the crystallographic orientation of the surface of the bufferlayer is directly related to the crystallographic orientation of thesurface of the substrate, and the crystallographic orientation of thelayer of superconductor material is directly related to thecrystallographic orientation of the surface of the buffer layer.Therefore, the superconducting properties exhibited by a multi-layersuperconductor having a buffer layer can depend upon thecrystallographic orientation of the buffer layer surface.

The conventional processes used to grow buffer layers on metalsubstrates and achieve this transfer of texture include vacuum processessuch as pulsed laser deposition, sputtering, and electron beamevaporation. Such techniques are disclosed in, for example, A. Goyal, etal., “Materials Research Society Spring Meeting, San Francisco, Calif.,1996; X. D. Wu, et al., Appl. Phys. Lett. 67:2397, 1995). Solution phasetechniques, including metalorganic deposition (MOD), are also used togrow buffer layers. Such techniques are disclosed in, for example, S. S.Shoup et al., J. Am. Cer. Soc., Vol. 81, 3019; D. Beach et al., Mat.Res. Soc. Symp. Proc., vol. 495, 263 (1988); M. Paranthaman et al.,Superconductor Sci. Tech., vol. 12, 319 (1999); D. J. Lee et al.,Japanese J. Appl. Phys., vol. 38, L178 (1999) and M. W. Rupich et al.,I.E.E.E. Trans. on Appl. Supercon. vol. 9, 1527 (1999).

When using solution phase techniques to deposit epitaxial oxide films onoxidizable metal or metal alloy substrates, it is necessary to carry outthe process in a reducing atmosphere relative to the oxidation potentialof the metal or metal alloy of the substrate. This is illustrated inFIG. 3, which shows the metal/metal oxide phase stability line for Niand W as a function of temperature and oxygen pressure. Typically forsubstrates such as nickel or NiW alloys, a hydrogen/argon environment isemployed, e.g., 4%/96% H/Ar. See, U.S. Pat. No. 6,077,344. The use ofthe reducing environment typically requires higher temperatures, e.g.,of about 1000-1200° C., to completely decompose the organic componentsof the precursor film and to nucleate and crystallize the oxide phase.Although these conditions produce oxide films with epitaxial texture,the high temperatures frequently result in undesirable grain growth androughening of the oxide film surface. The surface roughness increases asthe thickness of the oxide layer increases. Also the high processingtemperatures increase the rate of metal diffusion through the bufferlayer causing potential contamination of the buffer and subsequentlayers.

Under oxidizing conditions, it may be possible to directly deposit atextured oxide buffer layer on a metal substrate, particularly if themetal is not overly sensitive to oxidation, e.g., nickel. See, forexample, U.S. Pat. No. 6,440,211. Direct growth of epitaxial bufferlayers on a metal substrate under even mildly oxidizing conditions hasnot been successful to date, however, because the metal surfaceinevitably forms a metal oxide layer which may have a crystal structuredifferent than the underlying metal and which disrupts epitaxial growth.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a process is provided by whichepitaxial layers are grown on metal substrates using a solution basedtechnique under conditions that reduce the potential for substrate metaloxidation, avoid grain growth and surface roughening, and provide asmooth surface morphology of the deposited layer. The process also canprovide solution-grown epitaxial layers with low surface roughness atthicknesses of up to 300 nm.

One approach for achieving these and other desirable features is tofirst deposit a textured seed layer on a metal surface under a reducingatmosphere. A subsequent layer(s) is deposited over the seed layer froma solution-based precursor in a moist, oxidizing environment. The seedlayer serves as an oxygen diffusion barrier to prevent oxidation of theunderlying metallic surface upon exposure to the more oxidizingenvironment in the subsequent deposition steps. The oxidizingenvironment promotes decomposition of the organic components of thesolution-based precursor and nucleation of the oxide crystals at lowtemperatures and conditions the surface of the epitaxially grown film toprovide an epitaxial film with a fine-grained, smooth surface. The lowtemperature, oxidizing environment also provides a “thick,” e.g., up toabout 300 nm, epitaxial metal oxide layer.

In one or more embodiments of the present invention, a multilayerarticle is prepared by depositing a first material on the surface of ametal substrate to form a biaxially textured seed layer of the firstmaterial, the first material being deposited under reducing conditionsrelative to the metal substrate; and thereafter epitaxially depositing asecond material over the seed layer, wherein the second material isdeposited from a solution-based precursor under moist oxidizingconditions that are more oxidizing than the reducing conditions used inthe deposition of the first material, and wherein the atmosphere of theoxidizing conditions comprises a water partial pressure in the range ofabout 4 torr to about 40 torr, or about 4 to 25 torr, or about 10 to 25torr.

By “biaxially textured” as used herein it is meant that a surface of alayer has crystal grains in close alignment with a direction in theplane of the surface and a direction perpendicular to the surface. By“epitaxially depositing” as used herein it is meant that the depositedlayer has a crystallographic orientation that is directly related to thecrystallographic orientation of the surface onto which the layer isdeposited.

In one or more embodiments of the present invention, the second materialis deposited under moist, oxidizing conditions that include heating inan oxygen-containing atmosphere having an oxygen partial pressure of atleast 10⁻²¹ atm or in some embodiments, about 10⁻²¹-10⁻¹² atm oxygen. Inone or more embodiments of the present invention, oxidizing conditionsare controlled by controlling the relative partial pressures of hydrogenand water. The ratio of water partial pressure to hydrogen partialpressure in one or more embodiments is in the range of about 0.1 toabout 1. The second material is deposited as a layer at a thickness ofat least 5 nm to up to about 300 nm. The layer possesses a smoothsurface morphology and is suitable for deposition of additional layerssuch as a superconductor oxide layer thereon.

In one or more embodiments of the present invention, the first materialalso is deposited using a solution-based precursor, and the seed layeris grown from the precursor under a reducing atmosphere. In one or moreembodiments, the reducing atmosphere has low oxygen partial pressures,i.e., less than about 10⁻¹⁵, and in some embodiments about 10⁻¹⁵-10⁻²¹atm oxygen. In one or more embodiments, oxygen partial pressure iscontrolled by controlling the relative amounts of hydrogen partialpressure and, optionally, water partial pressure of the reducingconditions. The ratio of water partial pressure to hydrogen partialpressure in one or more embodiments is in a range of about 0.01 to about0.5, or less than about 0.1.

In one or more embodiments of the present invention, the first materialis deposited using a vacuum or low pressure deposition technique in areducing environment.

One or more layers optionally are deposited onto the substrate accordingto one or more embodiments of the invention. Layers may be depositedbetween the first material (seed) layer and the second material layer.Layers also may be deposited after the second layer. Additional layersmay be processed using the moist oxidizing conditions of the presentinvention, or using conventional vacuum or low pressure depositiontechniques.

BRIEF DESCRIPTION OF THE DRAWING

The present invention is described with reference to the followingdrawings, which are presented for the purpose of illustration only andare not intended to be limiting of the invention.

FIG. 1 is a cross-sectional view of a typical superconductor article.

FIG. 2 is a cross-sectional view of a superconductor article accordingto one or more embodiments of the present invention.

FIG. 3 is a plot of the stability line for various metal/metal oxides asa function of oxygen pressure and temperature.

FIG. 4 is a plot of the stability line for various metal/metal oxides asa function of the ratio of water partial pressure to hydrogen partialpressure and temperature.

FIG. 5A-5B illustrate a series of atomic force microscopy (AFM)micrographs illustrating the surface morphology of a ceria layerprepared according to one or more embodiments of the present invention.

FIG. 6 is a x-ray diffraction (XRD) pattern of a La₂Zr₂O₇ film growndirectly on a NiW substrate in a forming gas environment.

FIG. 7 is a x-ray diffraction (XRD) pattern of a La₂Zr₂O₇ film growndirectly on a NiW substrate in a forming gas environment containing 4torr H₂O.

FIG. 8 is a x-ray diffraction (XRD) pattern of a second La₂Zr₂O₇ filmgrown in a forming gas environment containing 4 torr H₂O on a firstLa₂Zr₂O₇ layer having 400 texture.

DESCRIPTION OF THE INVENTION

The invention relates to a multilayer article 20, such as asuperconducting article, and methods of making it. The articles andmethods of one or more embodiments of the present invention aredescribed with reference to FIG. 2.

The method involves first depositing a biaxially textured seed layer 24on a metal substrate 22 surface under reducing conditions, andthereafter epitaxially depositing a subsequent layer(s) 26 onto the seedlayer in a humid oxidizing environment. One or more additional layers28, such as a superconductor material, a precursor to a superconductormaterial, a cap material, e.g., a buffer layer on which a superconductormaterial is formed, or further buffer material can be deposited onto thelayer 26. Additional layers may also be deposited between layers 24 and26. In one or more embodiments layer 26 is a barrier layer, i.e., alayer that acts to substantially reduce diffusion of constituents fromthe substrate into the superconductor material. In one or moreembodiments layer 26 is a cap layer; i.e., a layer on which an activelayer such as a superconductor material is deposited. In someembodiments, a multilayer superconductor article contains a seed layer,a barrier layer and a cap layer, in which either the barrier layer, thecap layer or both are deposited in a humid oxidizing environmentaccording to one or more embodiments of the present invention.

Substrates

Metal substrate 22 can be formed of any material capable of supporting abiaxially textured seed layer 24. Examples of suitable substratematerials include metals such as nickel, silver, copper, zinc, aluminum,iron, chromium, vanadium, palladium, molybdenum and their alloys. Insome embodiments, substrate 22 can be a superalloy. In certainembodiments, substrate 22 can be in the form of an object having arelatively large surface area (e.g., a tape or a wafer). In theseembodiments, substrate 22 is preferably formed of a relatively flexiblematerial.

In some embodiments, the substrate is a binary alloy that contains twoof the following metals: copper, nickel, chromium, vanadium, aluminum,silver, iron, palladium, molybdenum, tungsten, gold and zinc. Forexample, a binary alloy can be formed of nickel and chromium (e.g.,nickel and at most 20 atomic percent chromium, nickel and from aboutfive to about 18 atomic percent chromium, or nickel and from about 10 toabout 15 atomic percent chromium). As another example, a binary alloycan be formed of nickel and copper (e.g., copper and from about five toabout 45 atomic percent nickel, copper and from about 10 to about 40atomic percent nickel, or copper and from about 25 to about 35 atomicpercent nickel). As a further example, a binary alloy can contain nickeland tungsten (e.g., from about one atomic percent tungsten to about 20atomic percent tungsten, from about two atomic percent tungsten to about10 atomic percent tungsten, from about three atomic percent tungsten toabout seven atomic percent tungsten, about five atomic percenttungsten). In certain embodiments, the substrate contains more than twoof the above-listed metals (e.g., a ternary alloy or a quaternaryalloy).

In some embodiments, the alloy can contain one or more oxide formers(e.g., Mg, Al, Ti, Cr, Ga, Ge, Zr, Hf, Y, Si, Pr, Eu, Gd, Th, Dy, Ho,Lu, Th, Er, Tm, Be, Ce, Nd, Sm, Yb and/or La, with Al being thepreferred oxide former), as well as two of the following metals: copper,nickel, chromium, vanadium, aluminum, silver, iron, palladium,molybdenum, gold and zinc. The alloys can contain at least about 0.5atomic percent oxide former (e.g., at least about one atomic percentoxide former, or at least about two atomic percent oxide former) and atmost about 25 atomic percent oxide former (e.g., at most about 10 atomicpercent oxide former, or at most about four atomic percent oxideformer).

The above-described alloys and compositions can include relatively smallamounts of additional metals (e.g., less than about 0.1 atomic percentof additional metals, less than about 0.01 atomic percent of additionalmetals, or less than about 0.005 atomic percent of additional metals).

The alloy substrate can be produced by, for example, combining theconstituents in powder form, melting and cooling or, for example, bydiffusing the powder constituents together in solid state. The substratecan then be deformation processed (e.g., by annealing and rolling,swaging, extrusion and/or drawing) to form a textured surface (e.g.,biaxially textured or cube textured). Alternatively, the alloyconstituents can be stacked in a jelly roll configuration, and thendeformation textured. In some embodiments, a material with a relativelylow coefficient of thermal expansion (e.g., Nb, Mo, Ta, V, Cr, Zr, Pd,Sb, NbTi, an intermetallic such as NiAl or Ni₃Al, or mixtures thereof)can be formed into a rod and embedded into the alloy prior todeformation texturing.

Preferably, the surface of substrate 22 has a relatively well definedcrystallographic orientation. For example, the surface of substrate 22can be a biaxially textured surface (e.g., a (113)[211] surface) or acube textured surface (e.g., a (100)[011] surface or a (100)[001]surface). Preferably, the peaks in an X-ray diffraction pole figure ofsurface have a FWHM of less than about 20° (e.g., less than about 15°,less than about 10°, or from about 5° to about 10°).

The surface of substrate 22 can also be prepared using vacuum processes,such as ion beam assisted deposition, inclined substrate deposition andother vacuum techniques known in the art to form a biaxially texturedsurface on, for example, a randomly oriented polycrystalline surface. Incertain embodiments (e.g., when ion beam assisted deposition is used),surface of substrate 22 need not be textured (e.g., the surface can berandomly oriented polycrystalline, or the surface can be amorphous).

In some instances, the metal substrate includes an intermediate metal oralloy layer, typically prepared using a metal that is more stable tooxidation than the underlying metal substrate. Metals suitable for usein the present invention also include those epitaxial metal or alloylayers that do not form surface oxides when exposed to P₀₂ andtemperature conditions required during subsequent growth of epitaxialbuffer layers. Exemplary intermediate metal layers include nickel, gold,silver, palladium, and alloys thereof.

In some of these embodiments, the intermediate metal seed layer istransient in nature. “Transient,” as used herein, refers to anintermediate layer that is wholly or partly incorporated into or withthe biaxially textured substrate following the initial nucleation andgrowth of the epitaxial film. Even under these circumstances, theintermediate layer and biaxially textured substrate remain distinctuntil the epitaxial nature of the deposited film has been established.Care should be taken that the deposited intermediate metal layer is notcompletely incorporated into or does not completely diffuse into thesubstrate before nucleation and growth of the initial buffer layerstructure causes the epitaxial layer to be established. This means thatafter selecting the metal (or alloy) for proper attributes such asdiffusion constant in the substrate alloy, thermodynamic stabilityagainst oxidation under practical epitaxial growth conditions andlattice matching with the epitaxial layer, the thickness of thedeposited metal layer is adapted to the epitaxial layer depositionconditions, in particular to temperature. Deposition of the intermediatemetal layer can be accomplished using a vacuum process such asevaporation or sputtering, or by electrochemical means such aselectroplating (with or without electrodes). These depositedintermediate metal layers may or may not be epitaxial after deposition(depending on substrate temperature during deposition), but epitaxialorientation can subsequently be obtained during a post-deposition heattreatment.

In certain embodiments, sulfur can be formed on the surface of the metallayer. The sulfur can be formed on the surface of the metal layer, forexample, by exposing the intermediate layer to a gas environmentcontaining a source of sulfur (e.g., H₂S, a tantalum foil or a silverfoil) and hydrogen (e.g., hydrogen, or a mix of hydrogen and an inertgas, such as a 5% hydrogen/argon gas mixture). This can be performed atelevated temperature (e.g., at a temperature of from about 450° C. toabout 1100° C., from about 600° C. to about 900° C., or at about 850°C.). The pressure of the hydrogen (or hydrogen/inert gas mixture) can berelatively low (e.g., less than about one torr, less than about 1×10⁻³torr, less than about 1×10⁻⁶ torr) or relatively high (e.g., greaterthan about 1 torr, greater than about 100 torr, greater than about 760torr).

Without wishing to be bound by theory, it is believed that exposing thetextured substrate surface to a source of sulfur under these conditionscan result in the formation of a superstructure (e.g., a c(2×2)superstructure) of sulfur on the textured substrate surface. It isfurther believed that the superstructure can be effective in stabilizing(e.g., chemically and/or physically stabilizing) the surface of themetal layer. Moreover, it is believed that other superstructures mayalso be effective in stabilizing (e.g., chemically and/or physicallystabilizing) the surface. For example, it is believed that an oxygensuperstructure, a nitrogen superstructure, a carbon superstructure, apotassium superstructure, a cesium superstructure, a lithiumsuperstructure or a selenium superstructure disposed on the surface maybe effective in enhancing the stability of the surface.

The oxygen sensitivity of the metal substrate will vary, and the growthconditions of the subsequently deposited layers are adapted to reflectthe relative oxygen-sensitivity of the metal substrate. For example,thicker seed layers and/or less oxidizing conditions are employed insubsequent processing steps to avoid undesirable oxidation of the metalsubstrate.

Seed Layer

In one or more embodiments of the present invention, a seed layer 24 isformed on the surface of substrate 22. The role of the seed layer is toprovide a crystalline, biaxially textured surface for the nucleation andgrowth of additional epitaxially deposited layers and to inhibit theoxidation of the substrate during deposition of the additional layerswhen carried out in an oxidizing atmosphere relative to the substrate.The seed layer may also provide an epitaxial template for growth ofsubsequent oxide layers. In order to accomplish these goals, thebiaxially textured seed layer should grow over the entire surface of themetal substrate and be free of any contaminants that may interfere withthe deposition of subsequent epitaxial buffer layers.

The thickness of the deposited seed layer is adapted to the subsequentoxide layer deposition conditions, in particular to temperature andoxygen partial pressure. The seed layer is not required to be thick,although it should be sufficiently thick to inhibit or slow down oxygendiffusion during deposition of subsequent oxide layers. In one or moreembodiments of the present invention, the seed layer has a thickness ofgreater than about 20 nm. In other embodiments, the seed layer has athickness of about 25-200 nm, or about 50-80 nm.

The seed layer also is selected for proper attributes such as diffusionconstant in the substrate alloy, thermodynamic stability againstoxidation under practical epitaxial oxide layer growth conditions andlattice matching with the subsequently deposited epitaxial oxidelayer(s). Examples of materials suitable for use as a seed layer includemetals having a cubic symmetry, and metal oxides and metal nitrides thathave a good lattice match for the substrate. Exemplary materials includesilver, gold, palladium, nickel, CeO₂, Y₂O₃, TbO_(x), GaO_(x),yttria-stabilized zirconium (YSZ), LaAlO₃, SrTiO₃, Gd₂O₃, LaNiO₃,LaCuO₃, NdGaO₃, NdAlO₃, MgO, CaF₂, AlN, NbN, TiN, VN, YN, ZrN, NiO,Al₂O₃, SmBa₂Cu₃O_(x), MgF₂, LaMnO₃, La_(0.66)Ca_(0.33)MnO₃,La_(0.66)Sr_(0.33)MnO₃, La_(0.66)Ba_(0.33)Mn₃, La₂Zr₂O₇, Gd₂Zr₂O₇,Ba₂Zr₂O₇ and doped compounds thereof and/or nitrides as known to thoseskilled in the art.

The seed layer can be deposited using any known deposition technique,including low pressure deposition techniques such as sputtering andphysical vapor deposition (PVD), electron beam and thermal evaporation,pulsed laser deposition (PLD), and metal organic chemical vapordeposition (MOCVD). Solution coating processes can also be used fordeposition of the seed layer onto the substrate. Solution deposition istypically carried out at or near atmospheric pressures.

The seed layer is deposited under reducing conditions. As used herein“reducing conditions” relative to the metal substrate refers toprocessing conditions at which the oxygen partial pressure duringdeposition is less than that needed to form a stable oxide of one ormore metals of the metal substrate. The appropriate conditions are knownin the art and may vary depending on the metal/metal oxide system underconsideration. FIG. 3 provides phase stability lines for various metalsand their metal oxides as a function of oxygen partial pressure andtemperature. The metal is stable at oxygen partial pressures andtemperatures that are below the curve indicated for the respective metaloxides. Thus, reducing conditions can be determined for each metal/metaloxide system.

The reducing conditions may include a hydrogen partial pressure.Exemplary reducing atmospheres include 4% v/v hydrogen in argon, helium,or nitrogen being a suitable gas composition. Mixtures of 2-6% v/vhydrogen are commonly referred to as “forming gas” and are not generallycombustible under most conditions. Carbon monoxide/carbon dioxidemixtures are also commonly used as gaseous reducing agents. The totaloxygen partial pressure is desirably maintained below 10⁻¹⁵, and in someembodiments is in the range of about 10⁻¹⁵ to about 10⁻²¹ atm. In otherexemplary embodiments, the partial pressure of oxygen is maintained atlow levels using a mixture of hydrogen gas and water vapor. The ratio ofthe two is selected to maintain the reducing environment needed for thedeposition of the seed layer without oxidation of the metal substrate.The appropriate ratio of water partial pressure (P_(H20)) to hydrogenpartial pressure (P_(H2)) or oxygen partial pressure (P₀₂) will vary foreach metal/metal oxide system.

FIG. 4 provides phase stability lines for several metals (and theiroxides) that are typically used as substrates in the multilayer articlesof the present invention. The more stable the metal to oxidation, thehigher the curve and the greater the permissible amount of water in thedeposition atmosphere. In one or more embodiments using a tungstencontaining substrate, the ratio of water partial pressure to hydrogenpartial pressure is in the range of about 0.01 to about 0.5, or lessthan about 0.1, at temperatures between about 400° C. to about 1100° C.An inert gas can also be included in the processing atmosphere.

As noted above, the seed layer may be deposited using a solution-baseddeposition technique. As used herein, “solution-based deposition” refersto a technique that deposits a precursor of a target material from aliquid medium in which the precursor is dissolved, suspended orslurried. The precursor is then treated under suitable conditions, e.g.,suitable temperatures and atmospheres, to obtain the target material.Various methods may be used in preparing a seed layer using asolution-based deposition process according to one or more embodimentsof the present invention.

Suitable solution preparation techniques include sol-gel processes thatuse metal alkoxide complexes in alcohol solution, or metal-organicdecomposition (MOD) techniques that use metal organic precursors in anorganic solvent, water, or an aqueous solution. In one or moreembodiments of the present invention, the precursor solution usesalkoxide, carboxylate, or nitrate compounds of the appropriate metals.The coating solution can be prepared using these solutions or any othercoating solution capable of being coated on a metal substrate andsubsequently capable of forming a metal or a metal oxide or a metalnitride on the substrate.

Any method of applying the coating solution to the metal substrate isacceptable for use with this invention. Exemplary methods of applyingthe coating solution to the metal substrate include spin coating, dipcoating, slot die coating, gravuere coating and ink jet coating. For anyof the methods, the metal substrate can be coated in a controlledatmosphere or in air. Spin coating involves spinning the metal substrateat high revolutions per minute (RPM), for example approximately 2,000RPM, and applying the solution onto the metal substrate. Any spinnercapable of applying a coating solution to the metal substrate isacceptable for use with this invention. Slot die coating involvesapplying a liquid to a substrate, in which the liquid is forced from areservoir through a slot by pressure, and is transferred to a movingsubstrate. So long as coating solution is applied to the metal substratewith the desired thickness and uniformity, the invention is not limitedas to any particular coating process.

The coating solution is heated under reducing conditions to decomposethe precursor, thereby leaving the corresponding metal oxide (or metalor metal nitride) remaining on the metal substrate. The reducingatmosphere also prevents any oxidation of the surface of the metalsubstrate. The article is heated for a combination of time andtemperature sufficient to decompose the organic components of theprecursor solution and to leave the desired crystal structure of themetal, metal oxide or metal nitride. Any time, temperature andatmosphere combination that accomplishes these goals is acceptable foruse with the invention. See, for example, FIGS. 3 and 4 for guidance inselecting the appropriate reducing conditions according to one or moreembodiments of the present invention. According to one or moreembodiments of the present invention, temperatures can range from about800° C. to about 1200° C., or from about 1000° C. to about 1100° C.under an oxygen partial pressure of less than about 10⁻¹⁵ atm, or lessthan about 10⁻¹⁶ atm, or less than about 10⁻¹⁷ atm, or less than about10⁻¹⁸ atm, or less than about 10⁻¹⁹ atm, or less than about 10⁻²⁰ atm,or less than about 10⁻²¹ atm.

In some embodiments, a biaxially textured seed layer 24 can be formed onan amorphous substrate using ion beam assisted deposition (IBAD). Inthis technique, a seed layer material is evaporated using, for example,electron beam evaporation, sputtering deposition, or pulsed laserdeposition while an ion beam (e.g., an argon ion beam) is directed at asmooth amorphous surface of a substrate onto which the evaporated seedlayer material is deposited. For example, the seed layer can be formedby ion beam assisted deposition by evaporating a seed layer materialhaving a rock-salt like structure (e.g., a material having a rock saltstructure, such as an oxide, including MgO, or a nitride) onto a smooth,amorphous surface (e.g., a surface having a root mean square roughnessof less than about 100 Angstroms) of a substrate so that the seed layermaterial has a surface with substantial alignment (e.g., about 13° orless), both in-plane and out-of-plane.

The conditions used during IBAD deposition of the seed layer materialcan include, for example, a substrate temperature of from about 0° C. toabout 750° C. (e.g., from about 0° C. to about 400° C., from about roomtemperature to about 750° C., from about room temperature to about 400°C.), a deposition rate of from about 1.0 Angstrom per second to about4.4 Angstroms per second, an ion energy of from about 200 eV to about1200 eV, and/or an ion flux of from about 110 microamperes per squarecentimeter to about 120 microamperes per square centimeter.

These methods are described in PCT Publication No. WO 99/25908,published on May 27, 1999, and entitled “Thin Films Having ARock-Salt-Like Structure Deposited on Amorphous Surfaces,” which ishereby incorporated by reference.

Second Layer

After the seed layer is deposited at a thickness sufficient to inhibitoxidation of the metal substrate and to provide a stable crystallinesurface, a second layer 26 of the same or different material can bedeposited under more oxidizing conditions using a solutions-basedprecursor. The second layer may be deposited directly onto the seedlayer, or it may be deposited onto intermediate layers that have beendeposited onto the seed layer. The second layer is typically an oxidebuffer layer, or an oxide cap layer onto which an active layer isdeposited.

The solution-based precursor provides a simple and rapid method ofdepositing relatively thick films, e.g., up to about 300 nm, or about 5nm to about 300 nm, or about 25 nm to 300 nm, or about 50 nm to 300 nm,or about 100 nm to about 300 nm; and the moist, oxidizing conditionspermit the rapid formation of dense, epitaxial metal oxide layers atlower temperatures and with a smooth surface. As used herein, a “smoothsurface” refers to a surface of the buffer layer that is relativelyatomically flat. In one or more embodiments, the surface roughness ofthe deposited layer is less than 10 nm rms, or about 1 to about 10 nmrms. The surface may also contain a high number of termination planesthat are oriented in a desirable direction. “Termination plane” of alayer refers to the plane of the layer that forms an interface withanother layer that is formed thereon.

In one or more embodiments, the second oxide layer has a terminationplane with a relatively high degree of orientation in a desirabledirection (e.g., (001) or (111)). At least about 25% (e.g., at leastabout 30%, at least about 40%, at least about 50%, at least about 60%,at least about 70%, at least about 80%, at least about 90%, at leastabout 95%) of the termination plane of the layer of the second oxidematerial is oriented in a desirable direction, e.g., the (001) plane orthe (111) plane. In one or more embodiments, a ratio of the amount ofthe termination plane of the layer of the second material that is the(111) plane to the amount of the termination plane of the secondmaterial that is the (001) plane is less than about one (e.g., less thanabout 0.75, less than about 0.5, less than about 0.4, less than about0.3, less than about 0.2, less than about 0.1).

In one or more embodiments, the buffer layer is ceria (CeO₂) and thetermination plane is oriented in the (001) direction. The terminationplane of the ceria buffer layer can have at least one (001) plateau withan area of about 500 nm² (as measured by atomic force microscopy) andcan be parallel to the termination plane of the substrate to withinabout 5 degrees.

Examples of materials suitable for use as a second layer include metaloxides that have a good lattice match for the substrate and the seedlayer. Exemplary materials include CeO₂, Y₂O₃, TbO_(x), GaO_(x), YSZ,LaAlO₃, SrTiO₃, Gd₂O₃, LaNiO₃, LaCuO₃, NdGaO₃, NdAlO₃, MgO, NiO, Ag,Al₂O₃, SmBa₂Cu₃O_(x), MgF₂, LaMnO₃, La_(0.66)Ca_(0.33)MnO₃,La_(0.66)Sr_(0.33)MnO₃, La_(0.66)Ba_(0.33)Mn₃O₃, La₂Zr₂O₇, Gd₂Zr₂O₇,Ba₂Zr₂O₇, Y₂Ce₂O₇ and doped compounds thereof as known to those skilledin the art.

The second layer is prepared using solution deposition techniques,including metalorganic deposition and/or sol-gel deposition, which areknown to those skilled in the art. The precursor solution can be anymetal source that is soluble in the organic or aqueous solution used toprepare the precursor solution. In one or more embodiments of thepresent invention, the precursor solution uses metal alkoxides oracetate or nitrate salts, metal oxides, metal hydroxides, metal halidesmetal acetylacetonates, metal carbonates, metal carboxylates or metaloxalates of the appropriate metals. The solvent or combination ofsolvents used in the precursor solution can include any solvent orcombination of solvents capable of dissolving the metal salts. Suchsolvents include, for example, alcohols, including methanol, ethanol,isopropanol and butanol, and water.

In one or more embodiments, the precursor solution is prepared usingmetal acetates. Metal acetates are inexpensive, readily available and/oreasy to prepare, however, many of the metal acetates have limitedsolubility in the precursor solvents. It has been discovered that thesolubility of metal acetates can be improved by reducing or eliminatingthe waters of hydration in the starting material and/or by complexingthe metals with higher molecular weight acid ligands, such as propionicacid. These acetate-derived precursor solutions are suitable forproducing epitaxial oxide films on textured surfaces. These methods andsolutions are described in U.S. Published Patent Application No.2002/0056401.

The second material is deposited from the precursor solution, which canbe made and applied as described herein above. For example, theprecursor solution can be applied using spin coating, dip coating, slotdie coating, gravuere coating, or ink jet coating, as described abovefor the seed layer.

If the precursor solution used to make the oxide layer does notadequately cover the underlying surface, then the resultant metal oxidelayer will not provide the desired protection of the underlying layersfrom oxygen or chemical diffusion during subsequent deposition steps andwill not provide a complete template for the epitaxial growth ofsubsequent layers. In one or more embodiments, the formation of thesecond layer is carried out so as to promote wetting of an underlyingsurface and to improve the coverage of the resultant metal oxide layer.The formation of metal oxide layers can be carried out using metalalkoxide precursors (for example, “sol gel” precursors), which areheated prior to decomposition. Substantially complete surface coverageis obtained by heating a sol gel film, and thereby allowing theprecursor to flow into the substrate grain boundary areas. The heatingcan be relatively low temperature, for example, from about 80° C. toabout 320° C., for example, from about 100° C. to about 300° C., or fromabout 100° C. to about 200° C. Such temperatures can be maintained fromabout 1 to about 60 minutes, for example, from about 2 to about 45minutes, or from about 15 to about 45 minutes. The heating step can alsobe carried out using higher temperatures for a shorter time, forexample, a film can be processed within two minutes at a temperature of300° C. This heating step can be carried out after, or concurrentlywith, the drying of excess solvent from the sol gel film. It must becarried out prior to decomposition of the film, however.

According to one or more embodiments of the present invention, aftercoating the seed (or other underlying) layer with the metal saltsolution, the layer can be air dried and then heated in an oxidizingdecomposition step. During decomposition of the precursor coating andformation of the metal oxide layer, the environment is oxidizingrelative to the reducing atmosphere used in the deposition of the seedlayer. The article is heated for a combination of time and temperaturesufficient to decompose the organic components of the precursor solutionand to provide the desired crystal structure of the metal oxide. Thenucleation step requires from about less than 1 minute to about 60minutes, or 1 to 30 minutes, or 1 to 15 minutes, to take place undertypical conditions; however, any time, temperature and atmospherecombination that accomplishes these goals is acceptable for use with theinvention.

In one or more embodiments, the oxidizing environment includes an oxygenpartial pressure greater than 10⁻²¹ atm, or in some embodiments in therange of 10⁻¹² to 10⁻²¹ atm. Water vapor is used to provide the desiredoxygen partial pressure. An inert gas may also be included in thedeposition environment. Although not bound by any particular mode ofoperation, it is believed that the use of water vapor provides theability to finely control the oxygen content in the processingenvironment. At elevated temperatures, the water vapor breaks down toprovide atomic (or molecular) oxygen at a predictable level. The amountof water vapor used to provide a desired oxygen level is greater thanthe corresponding amount of oxygen gas needed to provide the same oxygenlevel, and is therefore easier to control.

Oxygen acts as a catalyst for decomposing the coating solution at lowertemperatures. Thus, introduction of water or oxygen gas into theatmosphere advantageously allows for a lower processing temperature. Dueto higher oxygen partial pressures, e.g., greater than 10⁻²¹ atm, orgreater than about 10⁻²⁰ atm, or greater than about 10⁻¹⁹ atm, orgreater than about 10⁻¹⁸ atm, or greater than about 10⁻¹⁷ atm, orgreater than about 10⁻¹⁶ atm, or greater than about 10⁻¹⁵ atm, orgreater than about 10⁻¹⁴ atm, or greater than about 10⁻¹³ atm, or about10⁻²¹ to about 10⁻¹² atm, the organic components of the precursorsolution can be decomposed at lower temperatures than under the reducingconditions used for the deposition of the seed layer. Typically,processing temperatures can be lowered by about 100° C. to about 200° C.In one or more embodiments, the second layer is heated at temperaturesin the range of about 700° C. to about 1200° C. at oxygen partialpressures in the range of about 10⁻²¹ to 10⁻¹² atm.

The oxygen partial pressure is a function of the temperature. The higherthe processing temperature, the higher the oxgyen partial pressure thatcan be used in both the first and second layer deposition. For a NiWsubstrate, the ranges for pO2 are somewhat temperature dependent, andcan range as follows: P_(O2) at 800° C. P_(O2) at 1100° C. First layer<10⁻²¹ atm <10⁻¹⁵ atm Second layer 10⁻²¹-10⁻¹⁸ atm 10⁻¹⁶-10⁻¹² atmThus, oxidizing conditions can exist over a range of oxygen (and water)partial pressures, so long as the temperature is adjusted accordingly.

The ratio of hydrogen gas and water is used to control the oxygencontent of the processing environment to form metal oxide layer. In oneor more embodiments of the present invention, the ratio of the partialpressure of water to the partial pressure of hydrogen during depositionof the second material is in the range about 0.2 to about 1 at atemperature of about 800° C. to about 1100° C. The water partialpressure can be in the range of about 10-40 torr or about 10-25 torr, orabout 4-25 torr. The second layer is heated in an oxidizing environmentincluding a water partial pressure in the range of about 10 torr toabout 40 torr and a hydrogen partial pressure in the range of about 20torr to about 200 torr. The balance of the gas is an inert carrier gassuch as nitrogen or argon in an amount to provide a total pressure inthe range of about 30 torr to about 760 torr. In one or moreembodiments, the second layer is formed at substantially atmosphericpressures.

Suitable conditions for the epitaxial deposition of the buffer layer isselected in consideration with the chemical composition of theunderlying layers, in particular the seed layer, and the oxygensensitivity of the metal substrate. Generally, the seed layer serves toslow down, but not eliminate, oxygen diffusion through the seed layer.Higher oxygen levels, while possibly desirable for optimal processing ofthe buffer layer, shorten the available processing time beforeundesirable oxidation of the underlying metal substrate (and possibleconcomitant loss of epitaxy and other undesirable processingconsequences) occur. Thus, oxidizing conditions for deposition of thebuffer layer are limited at the upper end. Under most circumstances, theoxygen partial pressure is less than about 10⁻¹² atm. When using watervapor as the oxygen source, the partial pressure of water vapor istypically less than about 40 torr. In one or more embodiments, thepartial pressure of water vapor is about 10-25 torr, or about 4-25 torr.

The high oxygen content permits the growth of high quality films (e.g.,epitaxial growth, high density and smooth surface morphology) to muchgreater thicknesses than is possible under more reducing conditions.Under the reducing conditions described above, buffer layers havingcomparable film properties, e.g., rms of about 10 nm, cannot be obtainedat thicknesses greater than about 20-30 nm. In comparison, oxide layerscan be grown to thicknesses up to about 300 nm when using the moist,oxidizing conditions according to one or more embodiments of the presentinvention.

Additionally, in one or more embodiments of the present invention, theformation of metal oxide layers can be carried out under conditions atwhich the level of carbon contamination is greatly reduced over otherknown solution-based deposition processes. The carbon contaminationaccompanying conventional oxide film preparation in a reducingenvironment (e.g., 4% H₂—Ar) is believed to be the result of anincomplete removal of the organic components of the film. The presenceof carbon-containing contaminants C_(x)H_(y) and C_(a)H_(b)O_(c) in ornear the oxide layer can be detrimental, since they can alter theepitaxial deposition of subsequent oxide layers. Preferably, thecarbon-containing contaminants are oxidized and removed from the filmstructure as CO₂ as the decomposition occurs. The deposition and growthof the buffer layer under moist oxidizing conditions provides theadditional benefit of combusting the carbon contaminants of theprecursor solution during film deposition and thereby reducing thecarbon levels in the deposited layer.

Moreover, the high oxygen content of the oxidizing environmentpositively affects the surface morphology of the deposited film toprovide a smooth surface. The use of water vapor in the oxidizingenvironment provides a surprising additional advantage of producingoxide films of smooth surface morphology, as has been described above.It is believed that the water vapor provides gas phase chemical speciescapable of affecting a change in the surface properties of the layer.

In one or more embodiments, the oxide layer has an atomically smoothsurface with a surface roughness of about 10 nm rms or a range of about2 nm to about 10 nm rms. In one or more embodiments, the oxide layer hasa low incidence of termination planes (e.g., less than 50%, andtypically less than about 40%, or about 30%, or about 20%, or about 10%or about 5%, or about 2%) other than the desired orientation. Exemplarydesired orientations typically include (001), (010) and (100); however,surfaces having termination planes of other orientations, e.g., (111),(110), (211), (201) and (301), can be obtained.

Variations of the invention will be apparent to those of skill in theart and are contemplated as within the scope of the invention. Forexample, a coating deposited from solution can be initially heated in adecomposition step under an atmosphere that is reducing relative to themetal substrate to form a seed layer. Once the oxide layer nucleates onthe metal substrate in the desired epitaxial orientation, the oxygenlevel of the processing gas is increased, for example, by adding watervapor or oxygen. Thus the seed layer and the second layer are preparedin a single, multi-stage heat treatment using the same precursorsolution. This process is demonstrated in Example 5.

In one or more embodiments, various combinations of layer materialsand/or layer thicknesses can be used. Some or all of these layers can bedeposited using moist oxidizing conditions according to one or moreembodiments of the present invention. By way of example only, a layer ofY₂O₃ or CeO₂ (e.g., from about 20 nanometers thick to about 50nanometers thick) is deposited onto seed layer 24; a layer of YSZ (e.g.,from about 0.1 micrometer thick to about 0.5 micrometer thick) isdeposited on the Y₂O₃ or CeO₂ surface; and a CeO₂ layer (e.g., about 20nanometers thick) is deposited onto the YSZ surface. One or more ofthese layers can be deposited using moist oxidizing conditions accordingto one or more embodiments of the present invention. Alternately,conventional deposition techniques, e.g., sputtering, IBAD, physicalvapor deposition, chemical vapor deposition, and physical laserdeposition, and the like, can be employed.

Additional Layers

Additional layers may be deposited on the second layer. For example, asuperconductor oxide layer can be deposited. The superconductor may be arare earth oxide superconductor material, such as yttrium barium copperoxide (“YBCO”). Various combinations of buffer layers and superconductormaterial layers can be used. For example, multiple buffer layers can bedisposed between the substrate and the superconductor layer. As anotherexample, multiple layers of superconductor material can be used. As anadditional example, combinations of buffer layers and superconductorlayers (e.g., alternating buffer layers and superconductor layers) canbe used.

The well-aligned smooth surface of the oxide layer deposited under humidoxidizing conditions can be used as a substrate for the deposition of anoxide superconductor layer. Certain termination plane orientations,e.g., (001), have been associated with higher critical current densityfor subsequently deposited oxide superconducting films. CeO₂ has beendemonstrated to form a high number of (001) planes when depositedaccording to one or more embodiments of the present invention. Thus aCeO₂ layer of high surface smoothness and well-aligned terminationplanes is deposited on an underlying substrate containing at least ametal substrate, e.g., NiW, and a seed layer, e.g., YSZ, according toone or more embodiments of the present invention. An oxidesuperconductor is deposited on the highly aligned termination planes ofthe CeO₂ layer using methods known to those of skill in the art. Theresultant multilayer oxide superconductor article possesses improvedcritical current density.

Typically, solution chemistry is used to prepare barium fluoride and/orother superconductor precursors; and a solution (e.g., a solutioncontaining metal salts, such as yttrium acetate, copper acetate, bariumacetate and/or a fluorinated acetate salt of barium) is disposed on asurface (e.g., on a surface of a substrate, such as a substrate havingan alloy layer with one or more buffer layers disposed thereon). Thesolution can be disposed on the surface using standard techniques (e.g.,spin coating, dip coating, slot coating). The solution is dried toremove at least some of the organic compounds present in the solution(e.g., dried at about room temperature or under mild heat), and theresulting material is reacted (e.g., decomposed) in a furnace in a gasenvironment containing oxygen and water to form barium fluoride and/orother appropriate materials (e.g., CuO and/or Y₂O₃). In someembodiments, the reactors noted above can be used in any or all of thesesteps.

Examples of metal salt solutions that can be used are as follows.

In some embodiments, the metal salt solution can have a relatively smallamount of free acid. In aqueous solutions, this can correspond to ametal salt solution with a relatively neutral pH (e.g., neither stronglyacidic nor strongly basic). The metal salt solution can be used toprepare multi-layer superconductors using a wide variety of materialsthat can be used as the underlying layer on which the superconductorlayer is formed.

The total free acid concentration of the metal salt solution can be lessthan about 1×10⁻³ molar (e.g., less than about 1×10⁻⁵ molar or about1×10⁻⁷ molar). Examples of free acids that can be contained in a metalsalt solution include trifluoroacetic acid, acetic acid, nitric acid,sulfuric acid, acids of iodides, acids of bromides and acids ofsulfates.

When the metal salt solution contains water, the precursor compositioncan have a pH of at least about 3 (e.g., at least about 5 or about 7).

In some embodiments, the metal salt solution can have a relatively lowwater content (e.g., less than about 50 volume percent water, less thanabout 35 volume percent water, less than about 25 volume percent water).

In embodiments in which the metal salt solution containstrifluoroacetate ion and an alkaline earth metal cation (e.g., barium),the total amount of trifluoroacetate ion can be selected so that themole ratio of fluorine contained in the metal salt solution (e.g., inthe form of trifluoroacetate) to the alkaline earth metal (e.g., bariumions) contained in the metal salt solution is at least about 2:1 (e.g.,from about 2:1 to about 18.5:1, or from about 2:1 to about 10:1).

In general, the metal salt solution can be prepared by combining solublecompounds of a first metal (e.g., copper), a second metal (e.g., analkaline earth metal), and a rare earth metal with one or more desiredsolvents and optionally water. As used herein, “soluble compounds” ofthe first, second and rare earth metals refer to compounds of thesemetals that are capable of dissolving in the solvent(s) contained in themetal salt solution. Such compounds include, for example, salts (e.g.,nitrates, acetates, alkoxides, iodides, sulfates and trifluoroacetates),oxides and hydroxides of these metals.

In certain embodiments, a metal salt solution can be formed of anorganic solution containing metal trifluoroacetates prepared frompowders of Ba(O₂CCH₃)₂, Y(O₂CCH₃)₃, and Cu(O₂CCH₃)₂ which are combinedand reacted using methods known to those skilled in the art. Forexample, the metal trifluoroacetate powders can be combined in a 2:1:3ratio in methyl alcohol to produce a solution substantially 0.94 M basedon copper content.

In certain embodiments, the metal salt solution can contain a Lewisbase. The rare earth metal can be yttrium, lanthanum, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium,cerium, praseodymium, neodymium, promethium, samarium or lutetium. Ingeneral, the rare earth metal salt can be any rare earth metal salt thatis soluble in the solvent(s) contained in the metal salt solution andthat, when being processed to form an intermediate (e.g., a metaloxyhalide intermediate), forms rare earth oxide(s) (e.g., Y₂O₃). Suchsalts can have, for example, the formulaM(O₂C—(CH₂)_(n)—CXX′X″)(O₂C—(CH₂)_(m)-CX′″X″″X′″″)(O₂C—(CH₂)_(p)—CX″″″X′″″″X″″″″)or M(OR)₃. M is the rare earth metal. n, m and p are each at least onebut less than a number that renders the salt insoluble in the solvent(s)(e.g., from one to ten). Each of X, X′, X″, X′″, X″″, X′″″, X″″″, X′″″″and X″″″″ is H, F, Cl, Br or I. R is a carbon containing group, whichcan be halogenated (e.g., CH₂CF₃) or nonhalogenated. Examples of suchsalts include nonhalogenated carboxylates, halogenated acetates (e.g.,trifluoroacetate, trichloroacetate, tribromoacetate, triiodoacetate),halogenated alkoxides, and nonhalogenated alkoxides. Examples of suchnonhalogenated carboxylates include nonhalogenated actetates (e.g.,M(O₂C—CH₃)₃). The alkaline earth metal can be barium, strontium orcalcium. Generally, the alkaline earth metal salt can be any alkalineearth metal salt that is soluble in the solvent(s) contained in themetal salt solution and that, when being processed to form anintermediate (e.g., a metal oxyhalide intermediate), forms an alkalineearth halide compound (e.g., BaF₂, BaCl₂, BaBr₂, BaI₂) prior to formingalkaline earth oxide(s) (e.g., BaO). Such salts can have, for example,the formula M′(O₂C—(CH₂)_(n)—CXX′X″)(O₂C—(CH₂)_(m)-CX′″X″″X′″″) orM′(OR)₂. M′ is the alkaline earth metal. n and m are each at least onebut less than a number that renders the salt insoluble in the solvent(s)(e.g., from one to ten). Each of X, X′, X″, X′″, X″″ and X′″″ is H, F,Cl, B or, I. R can be a halogenated or nonhalogenated carbon containinggroup. Examples of such salts include halogenated acetates (e.g.,trifluoroacetate, trichloroacetate, tribromoacetate, triiodoacetate).Generally, the transition metal is copper. The transition metal saltshould be soluble in the solvent(s) contained in the metal saltsolution. Preferably, during conversion of the precursor to theintermediate (e.g., metal oxyhalide), minimal cross-linking occursbetween discrete transition metal molecules (e.g., copper molecules).Such transition metals salts can have, for example, the formulaM″(CXX′X″—(CH₂)_(n)—CXX′X″) (O₂C—(CH₂)_(m)—CX′″X″″X′″″) or M″(OR)₂. M″is the transition metal. a and b are each at least one but less than anumber that renders the salt insoluble in the solvent(s) (e.g., from oneto five). Generally, n and m are each at least one but less than anumber that renders the salt insoluble in the solvent(s) (e.g., from oneto ten). Each of X, X′, X″, X′″, X″″, X′″″, X″″″, X′″″″, X″″″″, X′″″″″,X″″″″″, X′″″″″″ is H, F, Cl, Br or I. R is a carbon containing group,which can be halogenated (e.g., CH₂CF₃) or nonhalogenated. These saltsinclude, for example, nonhalogenated actetates (e.g., M″(O₂C—CH₃)₂),halogenated acetates, halogenated alkoxides, and nonhalogenatedalkoxides. Examples of such salts include copper trichloroacetate,copper tribromoacetate, copper triiodoacetate, Cu(CH₃COCHCOCF₃)₂,Cu(OOCC₇H₁₅)₂, Cu(CF₃COCHCOF₃)₂, Cu(CH₃COCHCOCH₃)₂,Cu(CH₃CH₂CO₂CHCOCH₃)₂, CuO(C₅H₆N)₂ and Cu₃O₃Ba₂(O—CH₂CF₃)₄. In certainembodiments, the transition metal salt is a carboxylate salt (e.g., anonhalogenated carboxylate salt), such as a propionate salt of thetransition metal (e.g., a nonhalogenated propionate salt of thetransition metal). An example of a nonhalogenated propionate salt of atransition metal is CU(O₂CC₂H₅)₂. In some embodiments, the transitionmetal salt is a simple salt, such as copper sulfate, copper nitrate,copper iodide and/or copper oxylate. In some embodiments, n and/or m canhave the value zero. In certain embodiments, a and/or b can have thevalue zero. An illustrative and nonlimiting list of Lewis bases includesnitrogen-containing compounds, such as ammonia and amines. Examples ofamines include CH₃CN, C₅H₅N and R₁R₂R₃N. Each of R₁ R₂, R₃ isindependently H, an alkyl group (e.g., a straight chained alkyl group, abranched alkyl group, an aliphatic alkyl group, a non-aliphatic alkylgroup and/or a substituted alkyl group) or the like. Without wishing tobe bound by theory, it is believed that the presence of a Lewis base inthe metal salt solution can reduce cross-linking of copper duringintermediate formation. It is believed that this is achieved because aLewis base can coordinate (e.g., selective coordinate) with copper ions,thereby reducing the ability of copper to cross-link.

Typically, the metal salt solution is applied to a surface (e.g., abuffer layer surface), such as by spin coating, dip coating, webcoating, slot coating, gravure coating, or other techniques known tothose skilled in the art, and subsequently heated.

The deposited solution is then heated to provide the superconductormaterial (e.g., YBCO). Without wishing to be bound by theory, it isbelieved that, in some embodiments when making YBCO, the solution isfirst converted to barium fluoride, the superconductor precursor isconverted to a BaO superconductor intermediate, and the BaOsuperconductor intermediate is then converted to YBCO.

In certain embodiments, formation of barium fluoride involves heatingthe dried solution from about room temperature to about 200° C. at arate of about 5° C. per minute in a nominal gas environment having atotal gas pressure of about 760 torr, and containing from about fivetorr to about 50 torr of water and from about 0.1 torr to about 760 torrof oxygen, with the balance inert gas (e.g., nitrogen, argon). Thetemperature is then ramped from about 200° C. to about 220° C. at a rateof at least about 1° C. per minute (e.g., at least about 5° C. perminute, at least about 10° C. per minute, at least about 15° C. perminute, at least about 20° C. per minute) while maintainingsubstantially the same nominal gas environment.

In certain of these embodiments, barium fluoride is formed by heatingthe dried solution in moist oxygen (e.g., having a dew point in therange of from about 20° C. to about 75° C.) to a temperature in therange of from about 300° C. to about 500° C.

In alternate embodiments, barium fluoride is formed by heating the driedsolution from an initial temperature (e.g., room temperature) to atemperature of from about 190° C. to about 215° C. (e.g., about 210° C.)in a water vapor pressure of from about 5 Torr to about 50 Torr watervapor (e.g., from about 5 Torr to about 30 Torr water vapor, or fromabout 10 Torr to about 25 Torr water vapor). The nominal partialpressure of oxygen can be, for example, from about 0.1 Torr to about 760Torr. In these embodiments, heating is then continued to a temperatureof from about 220° C. to about 290° C. (e.g., about 220° C.) in a watervapor pressure of from about 5 Torr to about 50 Torr water vapor (e.g.,from about 5 Torr to about 30 Torr water vapor, or from about 10 Torr toabout 25 Torr water vapor). The nominal partial pressure of oxygen canbe, for example, from about 0.1 Torr to about 760 Torr. This is followedby heating to about 400° C. at a rate of at least about 2° C. per minute(e.g., at least about 3° C. per minute, or at least about 5° C. perminute) in a water vapor pressure of from about 5 Torr to about 50 Torrwater vapor (e.g., from about 5 Torr to about 30 Torr water vapor, orfrom about 10 Torr to about 25 Torr water vapor) to form bariumfluoride. The nominal partial pressure of oxygen can be, for example,from about 0.1 Torr to about 760 Torr.

In other embodiments, heating the dried solution to form barium fluorideincludes one or more steps in which the temperature is heldsubstantially constant (e.g., constant within about 10° C., within about5° C., within about 2° C., within about 1° C.) for a relatively longperiod of time (e.g., more than about one minute, more than about fiveminutes, more than about 30 minutes, more than about an hour, more thanabout two hours, more than about four hours) after a first temperatureramp to a temperature greater than about room temperature. In theseembodiments, heating the metal salt solution can involve using more thanone gas environment (e.g., a gas environment having a relatively highwater vapor pressure and a gas environment having a relatively low watervapor pressure) while maintaining the temperature substantially constant(e.g., constant within about 10° C., within about 5° C., within about 2°C., within about 1° C.) for a relatively long period of time (e.g., morethan about one minute, more than about five minutes, more than about 30minutes, more than about an hour, more than about two hours, more thanabout four hours). As an example, in a high water vapor pressureenvironment, the water vapor pressure can be from about 5 Torr to about40 Torr (e.g., from about 25 Torr to about 38 Torr, such as about 32Torr). A low water vapor pressure environment can have a water vaporpressure of less than about 1 Torr (e.g., less than about 0.1 Torr, lessthan about 10 milliTorr, about five milliTorr).

In certain embodiments, heating the dried solution to form bariumfluoride can include putting the coated sample in a pre-heated furnace(e.g., at a temperature of at least about 100° C., at least about 150°C., at least about 200° C., at most about 300° C., at most about 250°C., about 200° C.). The gas environment in the furnace can have, forexample, a total gas pressure of about 760 Torr, a predetermined partialpressure of water vapor (e.g. at least about 10 Torr, at least about 15Torr, at most about 25 Torr, at most about 20 Torr, about 17 Torr) withthe balance being molecular oxygen. After the coated sample reaches thefurnace temperature, the furnace temperature can be increased (e.g., toat least about 225° C., to at least about 240° C., to at most about 275°C., to at most about 260° C., about 250° C.) at a predeterminedtemperature ramp rate (e.g., at least about 0.5° C. per minute, at leastabout 0.75° C. per minute, at most about 2° C. per minute, at most about1.5° C. per minute, about 1° C. per minute). This step can be performedwith the same nominal gas environment used in the first heating step.The temperature of the furnace can then be further increased (e.g., toat least about 350° C., to at least about 375° C., to at most about 450°C., to at most about 425° C., about 450° C.) at a predeterminedtemperature ramp rate (e.g., at least about 5° C. per minute, at leastabout 8° C. per minute, at most about 20° C. per minute, at most about12° C. per minute, about 10° C. per minute). This step can be performedwith the same nominal gas environment used in the first heating step.

In some embodiments, preparation of a superconductor material caninvolve slot coating the metal salt solution (e.g., onto a tape, such asa tape formed of a textured nickel tape having sequentially disposedthereon epitaxial buffer and/or cap layers, such as Gd₂O₃, YSZ andCeO₂). The coated metal salt solution can be deposited in an atmospherecontaining H₂O (e.g., from about 5 torr H₂O to about 15 torr H₂O, fromabout 9 torr H₂O to about 13 torr H₂O, about 11 torr H₂O) The balance ofthe atmosphere can be an inert gas (e.g., nitrogen). The total pressureduring film deposition can be, for example, about 760 torr. The film canbe decomposed, for example, by transporting the coated tape through atube furnace (e.g., a tube furnace having a diameter of about 2.5inches) having a temperature gradient. The respective temperatures andgas atmospheres of the gradients in the furnace, as well as thetransport rate of the sample through each gradient, can be selected sothat the processing of the film is substantially the same as accordingto the above-noted methods.

The foregoing treatments of a metal salt solution can result in bariumfluoride. Preferably, the precursor has a relatively low defect density.

In particular embodiments, methods of treating the solution can beemployed to minimize the formation of undesirable a-axis oriented oxidelayer grains, by inhibiting the formation of the oxide layer until therequired reaction conditions are attained.

While solution chemistry for barium fluoride formation has beendisclosed, other methods can also be used. For example, solid-state, orsemi solid state, precursor materials deposited in the form of adispersion. These precursor compositions allow for example thesubstantial elimination of BaCO₃ formation in final YBCO superconductinglayers, while also allowing control of film nucleation and growth. Twogeneral approaches are presented for the formulation of such precursorcompositions.

In one approach, the cationic constituents of the precursor compositionare provided in components taking on a solid form, either as elements,or preferably, compounded with other elements. The precursor compositionis provided in the form of ultrafine particles, which are dispersed sothat they can be coated onto and adhere onto the surface of a suitablesubstrate, intermediate-coated substrate, or buffer-coated substrate.These ultrafine particles can be created by aerosol spray, byevaporation or by similar techniques, which can be controlled to providethe chemical compositions and sizes, desired. The ultrafine particlesare less than about 500 nm, preferably less than about 250 nm, morepreferably less than about 100 nm and even more preferably less thanabout 50 nm. In general, the particles are less than about 50% thethickness of the desired final film thickness, preferably less thanabout 30% most preferably less than about 10% of the thickness of thedesired final film thickness. For example, the precursor composition cancomprise ultrafine particles of one or more of the constituents of thesuperconducting layer in a substantially stoichiometric mixture, presentin a carrier. This carrier comprises a solvent, a plasticizer, a binder,a dispersant, or a similar system known in the art, to form a dispersionof such particles. Each ultrafine particle can contain a substantiallycompositionally uniform, homogeneous mixture of such constituents. Forexample, each particle can contain BaF₂, and rare-earth oxide, andcopper oxide or rare earth/barium/copper oxyfluoride in a substantiallystoichiometric mixture. Analysis of such particles would desirablyreveal a rare-earth:barium:copper ratio as substantially 1:2:3 instoichiometry, with a fluorine:barium ratio of substantially 2:1 instoichiometry. These particles can be either crystalline, or amorphousin form.

In a second approach, the precursor components can be prepared fromelemental sources, or from a substantially stoichiometric compoundcomprising the desired constituents. For example, evaporation of a solidcomprising a substantially stoichiometric compound of desired REBCOconstituents (for example, YBa₂Cu₃O_(7-x)) or a number of solids, eachcontaining a particular constituent of the desired final superconductinglayer (for example, Y₂O₃, BaF₂, CuO) could be used to produce theultrafine particles for production of the precursor compositions.Alternatively, spray drying or aerosolization of a metalorganic solutioncomprising a substantially stoichiometric mixture of desired REBCOconstituents could be used to produce the ultrafine particles used inthe precursor compositions. Alternatively, one or more of the cationicconstituents can be provided in the precursor composition as ametalorganic salt or metalorganic compound, and can be present insolution. The metalorganic solution can act as a solvent, or carrier,for the other solid-state elements or compounds. According to thisembodiment, dispersants and/or binders can be substantially eliminatedfrom the precursor composition. For example, the precursor compositioncan comprise ultrafine particles of rare-earth oxide and copper oxide insubstantially a 1:3 stoichiometric ratio, along with a solublizedbarium-containing salt, for example, barium-trifluoroacetate dissolvedin an organic solvent, such as methanol.

If the superconducting layer is of the REBCO type, the precursorcomposition can contain a rare earth element, barium, and copper in theform of their oxides; halides such as fluorides, chlorides, bromides andiodides; carboxylates and alcoholates, for example, acetates, includingtrihaloacetates such as trifluroracetates, formates, oxalates, lactates,oxyfluorides, propylates, citrates, and acetylacetonates, and, chloratesand nitrates. The precursor composition can include any combination ofsuch elements (rare earth element, barium, and copper) in their variousforms, which can convert to an intermediate containing a barium halide,plus rare earth oxyfluoride and copper(oxyfluoride) without a separatedecomposition step or with a decomposition step that is substantiallyshorter than that which may be required for precursors in which allconstituents are solubilized, and without substantial formation ofBaCO₃, and which can subsequently be treated using high temperaturereaction processes to yield an epitaxial REBCO film with T_(c) of noless than about 89K, and J_(c) greater than about 500,000 A/cm² at afilm thickness of 1 micron or greater. For example, for a YBa₂Cu₃O_(7-x)superconducting layer, the precursor composition could contain bariumhalide (for example, barium fluoride), yttrium oxide (for example,Y₂O₃), and copper oxide; or yttrium oxide, barium trifluoroacetate in atrifluoroacetate/methanol solution, and a mixture of copper oxide andcopper trifluoroacetate in trifluoroacetate/methanol. Alternatively, theprecursor composition could contain Ba-trifluoroacetate, Y₂O₃, and CuO.Alternatively, the precursor composition could contain bariumtrifluoroacetate and yttrium trifluoroacetate in methanol, and CuO.Alternatively, the precursor composition could contain BaF₂ and yttriumacetate and CuO. In some preferred embodiments, barium-containingparticles are present as BaF₂ particles, or barium fluoroacetate. Insome embodiments the precursor could be substantially a solublizedmetalorganic salt containing some or all of the cation constituents,provided at least a portion of one of the compounds containing cationconstituents present in solid form. In certain embodiments, theprecursor in a dispersion includes a binder and/or a dispersant and/orsolvent(s).

The precursor compositions can be applied to substrate or buffer-treatedsubstrates by a number of methods, which are designed to producecoatings of substantially homogeneous thickness. For example, theprecursor compositions can be applied using spin coating, slot coating,gravure coating, dip coating, tape casting, or spraying. The substrateis desirably uniformly coated to yield a superconducting film of fromabout 1 to 10 microns, preferably from about 1 to 5 microns, morepreferably from about 2 to 4 microns.

More details are provided in PCT Publication No. WO 01/08236, publishedon Feb. 1, 2001, and entitled “Coated Conductor Thick Film Precursor.”

In preferred embodiments, a superconductor layer is well-ordered (e.g.,biaxially textured in plane, or c-axis out of plane and biaxiallytextured in plane). In embodiments, the bulk of the superconductormaterial is biaxially textured. A superconductor layer can be at leastabout one micrometer thick (e.g., at least about two micrometers thick,at least about three micrometers thick, at least about four micrometersthick, at least about five micrometers thick).

The invention is described with reference to the following examples,which are presented for the purpose of illustration only.

EXAMPLE 1 Growth of Ceria (CeO₂) Buffer Layer on a Lanthanum Zirconate(LZO) Seed Layer

Deposition of a LZO seed layer. Deposition of LZO seed layer on a nickeltungsten (NiW) substrate is accomplished from a metal alkoxide precursorsolution. The solution is prepared from a stoichiometric mixture oflanthanum methoxyethoxide and zirconium methoxyethoxide inmethoxyethanol. The LZO precursor is deposited onto a biaxially texturedNiW substrate with a thickness of about 25-100 nm. The LZO precursor isdecomposed and the LZO phase is nucleated on the NiW substrate at atemperature of about 1100° C. in a formiing gas environment. The LZOforms an epitaxial layer under these conditions.

Deposition of a CeO₂ buffer layer. The CeO₂ buffer layer is deposited onthe surface of the LZO seed layer. The CeO₂ precursor can be a nitrateor acetate precursor or any other appropriate precursor from which CeO₂is formed. The CeO₂ precursor film is spin coated onto the LZO film at2000 rpm and decomposed within 5-15 min at 1000° C. The CeO₂ phase isnucleated on the LZO layer in a humid H₂/Ar environment having an H₂Opartial pressure of 22.5-40 torr in forming gas. In addition to loweringthe nucleation temperature of the CeO₂, the process conditions alsopromote the formation of a CeO₂ surface morphology that is relativelyatomically flat and has a high number of termination planes with thedesired orientation.

EXAMPLE 2 Growth of a CeO₂ Layer on a Substrate

A 0.2 M Ce solution is prepared by dissolving a 0.7 g of anhydrouscerium(III)acetate (Ce(OAc)₃) in 10 mL of a 97:3 methanol:propionic acidsolvent system. The solution was spin-coated at 2000 rpm onto two NiWsubstrates onto which had been previously deposited a layer of 50 nmY₂O₃ using e-beam evaporation as a deposition technique. The coatedlayers were heated for 15 minutes at 900° C. and 1100° C., respectively,in an atmosphere containing 3% water (P_(H2)O, 22.5 torr) to form a(002) textured CeO₂ layer of 70-100 nm thickness. The introduction of 3%water in the processing atmosphere retained (002) texture as wasdetermined by x-ray diffration. Pole figures confirmed that the CeO₂layer was in-plane textured.

Atomic force microscopy (AFM) was used to probe the surface morphologyof the CeO₂ layer. AFM micrographs in FIG. 5 a (for sample treated at1100° C.) and FIG. 5 b (for sample treated at 900° C.) clearly show thedesired termination layers of CeO₂ separated by up to 10 nm high stepswithin a 1×1 μm² area. The termination layers themselves are atomicallysmooth and allow for good nucleation of the subsequent layer. Thesurface roughness is well below 10 nm rms over a 1×1 μm² area.

EXAMPLE 3 Growth of a La₂Zr₂O₇ (LZO) Layer on a NiW Substrate in aReducing Atmosphere

The coating solution was prepared from a stoichiometric mixture of Lamethoxyethoxide (0.25 mole/liter) and Zr methoxyethodide (0.25mole/liter) in methoxyethanol. The coating solution was deposited onto abiaxially textured NiW substrate with a thickness of to yield a La₂Zr₂O₇film about 50 nm thick. The coating was decomposed by heating for 5-15minutes at about 1100° C. in a forming gas atmosphere. The La₂Zr₂O₇formed with a single 400 texture as shown in the x-ray diffraction (XRD)pattern in FIG. 6. The peak at approximately 52° (2θ) is the (400) peakof the NiW substrate, and the peak at approximately 33° (2θ) isattributed to the (400) peak of the LZO layer. Thus, an LZO isepitaxially deposited onto a metal surface under reducing conditions.

EXAMPLE 4 Growth of a La2Zr2O7 Layer on a NiW Substrate in OxidizingAtmosphere

A coating prepared as described in Example 3 was deposited on a NiWsubstrate. The coating was decomposed by heating for 5-15 minutes atabout 1100° C. in a forming gas atmosphere containing about 4 torr H₂O.The La₂Zr₂O₇ formed with a single 222 texture as shown in FIG. 7, asestablished by the absence of any peak at about 33° (2θ) and thepresence of the (222) peak at about 29(2θ). Thus, an epitaxial layer wasnot formed from by coating the precursor layer directly onto the metalsubstrate under oxidizing conditions.

EXAMPLE 5 Growth of a Second La₂Zr₂O₇ Layer on La₂Zr₂O₇/NiW Substrate

A first La₂Zr₂O₇ layer was prepared as described in Example 3. A secondcoating, prepared as described in Example 3 was deposited on the firstLa₂Zr₂O₇ layer. The second coating was decomposed by heating for 5-15minutes at about 1100° C. in a forming gas atmosphere containing about 4torr H₂O. The La₂Zr₂O₇ formed with a predominately a 400 texture asshown in FIG. 8.

Incorporation by Reference

The following documents are hereby incorporated by reference.

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1. A method of making a multilayer article, comprising: depositing afirst material on the surface of a metal substrate to form a biaxiallytextured seed layer of the first material, the first material beingdeposited under reducing conditions relative to the metal substrate; andepitaxially depositing a second material over the seed layer wherein thesecond material is deposited from a solution-based precursor to form asecond layer under second conditions that are more oxidizing than thereducing conditions used in the deposition of the first material, andwherein the atmosphere of the second conditions comprises a waterpartial pressure in the range of about 4 torr to about 40 torr.
 2. Amethod of making a multilayer article, comprising: depositing a firstmaterial on the surface of a metal substrate to form a biaxiallytextured seed layer of the first material, the first material beingdeposited under reducing conditions relative to the metal substrate; andepitaxially depositing a ceria layer having a termination plane over theseed layer, at least 25% of the termination plane of the ceria layercomprising the (001) plane, wherein the ceria layer is deposited from asolution-based precursor under second conditions that are more oxidizingthan the reducing conditions used in the deposition of the firstmaterial, and wherein the atmosphere of the second conditions comprisesa water partial pressure in the range of about 4 torr to about 40 torr.3. The method of claim 1 or 2, wherein the reducing conditions fordepositing the first material comprise an oxygen partial pressure lessthan about 10⁻¹⁵ atm.
 4. The method of claim 1 or 2, wherein thereducing conditions for depositing the first material comprise an oxygenpartial pressure less than about 10⁻²¹ atm.
 5. The method of claim 1 or2, wherein the reducing conditions for the deposition of the first layercomprises forming gas.
 6. The method of claim 5, wherein the reducingconditions for the first layer comprise heating at a temperature in therange of about 400° C. to about 1200° C.
 7. The method of claim 1 or 2,wherein reducing conditions for the first layer comprise heating in anatmosphere of hydrogen and water.
 8. The method of claim 7, wherein theratio of the partial pressure of water to the partial pressure ofhydrogen during formation the first material is in the range of about0.1 to about 0.5 at a temperature of about 800° C. to about 1100° C. 9.The method of claim 1 or 2, wherein the first layer is epitaxiallydeposited.
 10. The method of claim 1 or 2, wherein the first material isdeposited using a technique selected from the group consisting ofsputtering, e-beam deposition, ion beam assisted deposition, physicalvapor deposition, physical laser deposition, chemical vapor deposition,and metal organic deposition.
 11. The method of claim 1 or 2, whereinthe first material is selected from the group consisting of metals,metal oxides, and metal nitrides that have crystalline surfaces that arestable under the second deposition conditions that are oxidizingrelative to the underlying substrate.
 12. The method of claim 1 or 2,wherein the first material is selected from the group consisting silver,gold, palladium, nickel, CeO₂, Y₂O₃, TbO_(x), GaO_(x), YSZ, LaAlO₃,SrTiO₃, Gd₂O₃, LaNiO₃, LaCuO₃, NdGaO₃, NdAlO₃, MgO, CaF₂, AlN, NbN, TiN,VN, ZrN, YN, NiO, Ag, Al₂O₃, SmBa₂Cu₃O_(x), MgF₂, LaMnO₃,La_(0.66)Ca_(0.33)MnO₃, La_(0.66)Sr_(0.33)MnO₃, La_(0.66)Ba_(0.33)Mn₃,La₂Zr₂O₇, Gd₂Zr₂O₇, Ba₂Zr₂O₇ and doped compounds thereof.
 13. The methodof claim 1 or 2, wherein the second conditions for the second layercomprise an oxygen partial pressure in the range of about 10⁻²¹ atm toabout 10⁻¹² atm.
 14. The method of claim 1 or 2, wherein the waterpartial pressure is in the range of about 4 torr to about 25 torr. 15.The method of claim 1 or 2, wherein the water partial pressure is in therange of about 10 torr to about 25 torr.
 16. The method of claim 1 or 2,wherein the second conditions comprise heating the second material in anatmosphere comprising hydrogen and water.
 17. The method of claim 16,wherein second conditions comprise a total pressure in the range of 30torr to 760 torr and a water partial pressure in the range of about 4torr to about 40 torr and a hydrogen partial pressure in the range ofabout 20 torr to about 50 torr, the balance being an inert carrier gas.18. The method of claim 17, wherein the water partial pressure is in therange of about 4 torr to about 25 torr.
 19. The method of claim 16,wherein the ratio of the partial pressure of water to the partialpressure of hydrogen in deposition the second material is in the rangeabove about 0.1 to about 1 at a temperature of about 800° C. to about1100° C.
 20. The method of claim 16, wherein said second conditionscomprise heating at a temperature about 700° C. to about 1200° C. 21.The method of claim 1, wherein the second material forms a buffer layer.22. The method of claim 1, wherein the second material is selected fromthe group of metal oxides consisting of CeO₂, Y₂O₃, ThO_(x), GaO_(x),YSZ, LaAlO₃, SrTiO₃, Gd₂O₃, LaNiO₃, LaCuO₃, NdGaO₃, NdAlO₃, MgO, NiO,Ag, Al₂O₃, SmBa₂Cu₃O_(x), MgF₂, LaMnO₃, La_(0.66)Ca_(0.33)MnO₃,La_(0.66)Sr_(0.33)MnO₃, La_(0.66)Ba_(0.33)Mn₃O₃, La₂Zr₂O₇, Gd₂Zr₂O₇,Ba₂Zr₂O₇, and Y₂Ce₂O₇ and doped compounds thereof.
 23. The method ofclaim 1, wherein the second layer is deposited to a thickness of aboutfive nanometers to about 300 nm.
 24. The method of claim 2, wherein theceria layer is deposited to a thickness of about five nanometers toabout 300 nm.
 25. The method of claim 1, wherein the second layer has asurface smoothness in the range of about 1 nm to about 1 nm rms.
 26. Themethod of claim 2, wherein at least 75% of the termination plane of theCeO₂ layer comprises the (001) plane.
 27. The method of claim 1, furthercomprising depositing one or more intermediate layers between the firstseed layer and the second material layer.
 28. The method of claim 2,further comprising depositing one or more intermediate layers betweenthe first seed layer and the ceria layer.
 29. The method of claim 27 or28, wherein the intermediate layer is epitaxially deposited from asolution-based precursor under third conditions that are more oxidizingthan the reducing conditions used in the deposition of the firstmaterial, and the third conditions comprise a water partial pressure inthe range of about 4 torr to about 40 torr.
 30. The method of claim 27or 28, wherein the intermediate layer is deposited using a techniqueselected from the group consisting of sputtering, e-beam deposition, ionbeam assisted deposition, physical vapor deposition, physical laserdeposition, chemical vapor deposition, and metal organic deposition. 31.The method of claim 1, further comprising depositing a cap material as alayer on the second material.
 32. The method of claim 2, furthercomprising depositing a cap material as a layer on the ceria layer. 33.The method of claim 31 or 32, wherein the cap layer is epitaxiallydeposited from a solution-based precursor under third conditions thatare more oxidizing than the reducing conditions used in the depositionof the first material, and the third conditions comprise a water partialpressure in the range of about 4 torr to 40 torr.
 34. The method ofclaim 31 or 32, wherein the cap layer is deposited using a techniqueselected from the group consisting of sputtering, e-beam deposition, ionbeam assisted deposition, physical vapor deposition, physical laserdeposition, chemical vapor deposition, and metal organic deposition. 35.The method of claim 1, further comprising depositing a superconductormaterial on the second deposited layer.
 36. The method of claim 2,further comprising depositing a superconductor material on the CeO₂layer.
 37. The method of claim 35 or 36, wherein the superconductormaterial comprises a rare earth oxide superconductor material.
 38. Themethod of claim 35 or 36, wherein the rare earth oxide superconductormaterial comprises YBCO.
 39. The method of claim 1 or 2, wherein themetal substrate is selected from the group consisting of nickel, silver,copper, zinc, aluminum, iron, chromium, vanadium, palladium, molybdenumand their alloys.